Tarzan the Diaper Man

Testosterone shrinks when men become dads. Does that mean they're designed to nurture kids?

Men who provided more childcare have more testosterone in the morning than did men who provided less childcare

For years, we've been told that men and women evolved for different roles. Men hunted, fought, and prowled for sex, leaving women to raise the kids. Now we're being told a different story: Men, like women, are designed to nurture children.

Will Saletan writes about politics, science, technology, and other stuff for Slate. He’s the author of Bearing Right.

It's a nicer story, but it's just as simplistic. And the evidence is just as ambiguous.

The putative smoking gun for the new theory is a study just published in Proceedings of the National Academy of Sciences. It found that testosterone levels in young Filipino men fell 30 percent after they became fathers—more than double the reduction that occurred in childless bachelors of the same cohort during the same four-year period. There was a particularly big drop among men with infants. Furthermore, men who said they took care of their kids for at least three hours a day registered 20 percent less testosterone than did men who weren't involved in child-rearing.

4. Men are designed to help raise kids. "Fathers Wired to Provide Offspring Care," says the press release from the study's authors. "Our study shows that human fathers are biologically wired to help with the job."

Does it? The study shows that men are wired to produce less testosterone after their kids are born. There are several reasons, apart from caring or helping, why this might make evolutionary sense. For example:

A. Lower testosterone is healthier. As the authors note, testosterone is associated with heart disease and some cancers. That's reason enough for nature to lower your level once you've achieved the testosterone-assisted goal of fathering a child.

B. Lower testosterone reduces risky behavior. The authors cite substance abuse as an example. This is a definite advantage to your offspring, and it requires no helping or caring. You don't have to be Wonder Dad. Just don't get stoned all the time, take your kid hunting for wild pigs, or leave her in a hot car with the windows rolled up.

C. Lower testosterone reduces aggression. As a general rule, the most dangerous person in any household is the father. If you swaddle and play with your child, that's great. But first things first: Don't kill him or beat up your pregnant wife. Testosterone reduction is a good way to avoid this.

None of these alternative theories explains why spending more time with your kids lowers your testosterone more than just fathering a child does. But they don't have to explain that effect, because the study doesn't show it. In this paper, the authors report each man's testosterone level at two points: before and after he became a father. They don't report his testosterone before and after spending time with his kids. So while it's true that fathers who were more involved in childcare had lower testosterone than fathers who didn't, there's no longitudinal evidence that the childcare caused the lower testosterone, rather than the other way around.

The authors discount the possibility that lower testosterone causes paternal childcare, since pre-fatherhood testosterone levels didn't predict subsequent childcare involvement. But testosterone levels dropped radically as soon as the men became fathers. What was each man's testosterone level at that point? Without that information, we can't know whether lower testosterone preceded and predicted his degree of child care.

Testosterone affects and is affected by many things. It probably does adjust to environmental cues as men become mates and then fathers. But we're just beginning to explore how and why this happens. The new evolutionary-psychology theory we're being fed has less to do with earth-shattering evidence than with changes in our economy and culture. Women are gaining more respect and consideration. Wages have shrunk, so both parents have to work for pay. Men have to help out more at home, and they can't get away with cheating the way they used to. For a bunch of reasons, we need a more domestic and egalitarian theory of masculinity. And we're using this study to sell it.

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